It is known in the art to replace a natural hip joint with an artificial hip cup and stem replacement. Numerous artificial implants are available that can be installed to replace the natural joint with an artificial ball and socket combination. The medullary canal may be opened using a reamer to create a passage through the medullary canal in the upper end of the femur where a hip stem may be implanted. A stem or femoral component of an artificial implant is inserted into the reamed portion of the medullary canal in a secure seated position. Typically, femoral implants include a neck member that extends outward and away from the stem and terminates in a spherical knob for insertion into the acetabular implant of the hip in rotational contact throughout the three major orthogonal axes. The acetabular socket implants consist of a bowl or cup shaped device that is secured to the matching cup shaped dimension of the hip joint acetabulum.
Component malposition of the artificial implants has been recognized as an important source of problems or complications including fixation failure, limb length discrepancy, excessive wear, and dislocations. Dislocations may be related to component orientation, soft-tissue tension, or failure to restore hip biomechanics. [21, 22] Many of these factors are under the control of the surgeon. Ideally, the surgical intervention should reestablish the center of the hip joint to the anatomical center which typically will be the concentric center of the acetabulum. For certain reasons, the surgeon may desire to medialize this center or move the centrum to a more superior and medial non-anatomical position. Placement of the femoral component requires insertion of the body or stem of the implant into the medullary canal of the femur. The positional aspects of insertion relate to alterations of leg length and the amount of offset of the new artificial center of the femur compared to the original anatomical position. [12]
Change in leg length of the lower extremity can be determined by measuring: 1) the change of the artificial acetabular center from the normal anatomical center; and 2) the change of the artificial femoral center from the normal anatomical center. [24] The arthritic disease process with loss of the normal articular cartilage surfaces may diminish the leg length from normal. The surgeon then has several reasons to optimize leg length which includes restoring the leg to normal and to restore the length to match the contralateral leg. The femoral offset is the measure of distance from the anatomical center of the hip to an arbitrary position of the proximal femur such as the tip of the greater trochanter or a radiographic line that is centered on the anatomical axis of the proximal femoral shaft. [20] The femoral offset is increased during the operative intervention if the distance from the femoral center to the lateral reference is increased. Conversely, offset is diminished if the distance is decreased. The surgeon may desire to restore the offset that is determined to be normal or anatomical as this may best restore the normal functional muscle and soft tissue length. Biomechanical reasons may also lead to increasing the offset, as the lever arm for hip abduction may be made more favorable with this increase.
Another aspect of correct component position is the risk for component dislocation or disarticulation. This risk can be increased by malposition of the implants. [17] Adequate soft tissue tension is needed to keep the components in position and this will be decreased if the offset is inadequate. Positioning of both the acetabular and femoral components is another important factor determining stability. Excessive anteversion or retroversion of the acetabular component can lead to instability. Optimally, this position should be at about 40° of inclination from the transverse plane and 15° anteversion from the sagittal plane. One study demonstrated a four-fold increase in the potential for prosthetic dislocation if the position exceeded 10° from these optimal locations. [10] Another problem for acetabular position is the relationship of the pelvis to the longitudinal body plane. This also will affect the ultimate stability of the component by altering the acetabular component position. Femoral component version is less well understood but it has been recognized that adverse anteversion of both the cup and stem can lead to instability. With standard implant insertion through the conventional surgical approach, trial and final reduction or rearticulating of the prosthetic implants requires the surgeon or the assistant to manually distract the lower extremity to the extent that the devices are manipulated to the normal position. If this position is too long or too short, the final prosthetic position will to too tight or too loose, requiring adjustment. Adjustments could include driving the femoral prosthesis into the femoral intramedullary canal or placing a larger prosthesis that will tend to add length as the prosthesis cannot be advanced as far. This fitting process can become cumbersome, and an optimal scenario would include the use of a system that precisely determines the prosthetic position. [19] Approaching such precision would, therefore, tend to minimize the conventional ‘trial and error’ method of repeated prosthetic reductions.
Another innovation has been to substantially diminish the surgical exposure through a limited or minimally invasive approach. This evolutionary method decreases damage to the local tissues, decreases blood loss, and expedites patient recovery. [16] The method is more exacting because less anatomical structures are violated, at the expense of limited access of certain landmarks. The surgeon has more difficulty recognizing structures that guide typical bone resection. Computer assisted surgery (CAS) or navigation has emerged as a new method that will give the surgeon intraoperative positional information to improve component position. For minimally invasive surgery, the CAS system may enhance the understanding of the normal anatomy and allow precise direct placement of trial implants, minimizing the ‘trial and error’ method of optimizing prosthetic placement. [9] The basic components of a CAS system are the computer which will record and integrate three dimensional position data; a positional data gathering tool which could be optical, ultrasound, electromagnetic, or radio frequency; and the targeted rigid bodies that are defined to the computer by a referencing protocol. Dynamic rigid bodies may be a human anatomical structure, a surgical instrument, a joint prosthesis, or a positional marker. These rigid bodies are then portrayed in a ‘virtual’ three dimensional computer representation where the surgeon may readily understand the positional inter-relationships.
CAS of the hip joint for prosthetic arthroplasty initiated with methods that delineate the anterior plane of the pelvis. [18] The anterior pelvic plane is in the coronal plane of the human body and connects the points of the two anterior superior iliac spines and the pubic tubercles. This plane then describes an x-y-z axis that allows the computer to measure various points about the pelvis, such as the anatomical hip center, the acetabular center, the femoral center, femoral offset and the leg length. The first important application in orthopaedic surgery was using this framework to enable guidance of acetabular component insertion where the surgeon could monitor in real time the inclination and anteversion of acetabular prosthetic positioning. Various methods have been developed to measure and describe the anatomical points used by the surgeon. The hip femoral pivot method uses a referenced femur, which when moved in a circular fashion, will define the focus of the cone shaped movements as the anatomical hip center. [11] Another method allows the surgeon to use digital radiographs or images where the surgeon picks the anatomical center by assessing the center of the acetabulum and femoral head. A third method, the least squares method, picks the instant center based on a series of reference points that describe the floor of the acetabulum. Recent CAS systems have navigated femoral position in relation to the pelvic reference by establishing a point of reference on the proximal femur and then monitoring this position during the surgical procedure. Typical outputs may include leg length measure, femoral offset measure, and femoral stem anteversion measure. However, most recent CAS systems can only measure the beginning and final positions of the femur in relation to the pelvis and do not eliminate the ‘trial and error’ method of placing the implants and doing reductions to assess the effects of this positioning. [14, 15] Thus, methods of minimally invasive surgery (MIS) would welcome further advantages of improved CAS techniques in measuring anatomical and prosthetic objects. [13]
A device and a method for implanting artificial joint components are known from U.S. Pat. No. 5,995,738. This Digioia patent creates an artificial component model that allows three dimensional simulation of limb range of motion of the femur and acetabulum driving the surgical procedure. Based on this patient specific determination, the acetabulum component can then be guided to the resolved “optimal position”. The computer assisted surgical system used to track references fixed to patient bones and surgical instruments is described in greater detail in commonly assigned U.S. Pat. No. 6,315,659. That patent describes the ability of the CAS system to render the momentary positional data of the patient and that of surgical instruments and apparatus employed in the operation, visibly on the display terminal of the computer. It is known that some devices exist to measure intraoperatively stem position. [1, 4, 6] It is known that some devices exist to measure intraoperatively acetabular component position. [1, 2, 3, 5] It is known that some devices exist to measure both stem and acetabular relative positions when the hip is anatomically reduced. [1, 6] The Grimes patent registers the anatomical dimensions of the implants for image guided implant insertion. [8]
Prior approaches have not sought to directly measure the position of the femur and acetabulum independently of each other and based upon a unifying reference point which, in the case of the present invention, is the anatomical hip center.
Neither have others sought a single device to measure both stem and cup position, which offers the advantage of simplifying the ergonomics.
Nor have others sought a device intended to be used when the hip is dislocated, which would offer the advantage of accelerating the decision process, by avoiding reducing the hip to measure prosthesis implant positions then dislocating the hip to change the implant type if necessary.